Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and servo sectors. The servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the velocity of the actuator arm as it seeks from track to track.
When manufacturing a family of disk drives, the head/disk subsystem is typically tested to verify that a particular vendor's head will meet certain design criteria. For example, a vendor's head may be tested to measure a phenomena referred to as Wide-Area Track Erasure (WTE) due to inadequate shielding of the write element. The WTE effect is typically measured by DC erasing a large band of tracks (such as 8,000 tracks), writing a low frequency signal (25 MHz) to a target track within the band of tracks, and then measuring the signal noise in the adjacent tracks. However, there are phenomena other than WTE that need to be measured when evaluating the suitability of a particular vendor's head, or when selecting an operating write current for a particular vendor's head.